Add modern-era (2026-07-28) test-server preset (#1700) (#1703)

* Add modern-era (2026-07-28) test-server preset (#1700)

The era-aware connection model (#1626, PR #1697) added per-server
`protocolEra` and the era-aware Connection Info UI, but no bundled test
server negotiated the modern era — the modern happy path had only
unit/Storybook fixtures, no live coverage.

Serve the modern (2026-07-28) protocol era from the HTTP test server via
the SDK's `createMcpHandler`. A `modern` flag on `ServerConfig` (and a
`transport.modern` field in the JSON config) mounts the handler with a
dual-era `legacy: "stateless"` fallback by default, or `legacy: "reject"`
for modern-only strict serving. The web-standard `fetch` handler reuses
the existing `toWebRequest`/`writeWebResponse` conversion, so no new
`@modelcontextprotocol/node` dependency is needed.

An SDK v2 client negotiating `protocolEra: "auto" | "modern"` now reaches
the modern leg end-to-end: `getProtocolEra() === "modern"`,
`getNegotiatedProtocolVersion() === "2026-07-28"`, and a populated
`getDiscoverResult()`. Verified live in the Inspector — Connection Info
shows Era Modern / Session Sessionless / the Discovery section.

- Modern serving path + `listen`/`startModernHttp` helpers in
  test-server-http.ts; modern-leg teardown in stop().
- `modern` config field threaded through load-config/resolve-config with
  a streamable-http-only validation guard; modern-http.json demo config.
- Integration test covering auto/modern negotiation, the dual-era legacy
  fallback, a modern tools/call, and the strict `legacy: "reject"`
  rejection.

Closes #1700

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNdjEPKLG637X8YmhDiEk5

* Review nit: gate transport.modern guard on truthiness

A falsy `transport.modern` (e.g. `false`) is a no-op that `resolveConfig`
normalizes to `undefined`, so it shouldn't trip the streamable-http-only
guard. Gate on truthiness instead of `!== undefined` so only *enabling*
modern on a non-HTTP transport is rejected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XNdjEPKLG637X8YmhDiEk5

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -110,6 +110,8 @@ npm run test-servers:build   # (from clients/web) → tsc -p test-servers, emits
 
 The Vite alias `@modelcontextprotocol/inspector-test-server` (in `clients/web/vite.config.ts`) points at `test-servers/build/index.js` so `getTestMcpServerPath()` resolves to a real `.js` path.
 
+A streamable-HTTP server can also serve the **modern (2026-07-28) protocol era** via the SDK's `createMcpHandler` — set `transport.modern` in the JSON config (`true` for dual-era stateless serving, or `{ "legacy": "reject" }` for modern-only strict), or pass `modern` on the `ServerConfig` for an in-process `createTestServerHttp`. This is what lets an Inspector connection negotiating `protocolEra: "auto" | "modern"` reach the modern leg (populated `server/discover`, sessionless). See `test-servers/configs/modern-http.json`.
+
 ## Building
 
 ```bash